Intracellular microelectrodes are used to study the electrophysiology of single fibers from mammalian hearts in an attempt to determine the mechanisms responsible for disturbances of rhythm and conduction and the mode of action of cardiac drugs. Emphasis is being placed on studies on depressed fibers, on correllations between structure and function and on the effects of a variety of drugs on isolated Purkinje fibers perfused with blood from a donor animal.